Window glass material (Eevee)
The refraction algorithms in Eevee (which the glass shader uses either) lower resolution of everything behind refracted surface to speed up render and performance - this is what causes the bluriness. If you don't need a refraction distortion, the solution as it has been already mentioned is in using Principled BSDF. Increase "Metallic" all the way up to 1 and plug in Fresnel node into Alpha socket - you will get a fake glass material without lowering a background resolution. Anyway if you need a physically correct refraction I'm afraid the only way is to use Cycles.